U.S. Pat. No. 3,765,233 issued on Oct. 16, 1973 in the name of Reimar Germann shows circuit means for testing the compression in the cylinders of spark ignition engines. Included therein is a clamp type pickup 11 for generating an electrical pulse e when the No. 1 cylinder is firing. The pulse is applied to the first step 12.sub.1 of a shift register for synchronizing or starting purposes. The shift register output signals g, h, i and j trigger flip-flops 14.sub.1 - 14.sub.4 for different ones of the engine cylinders. Pickup 11 synchronizes the shift register with the engine so that waveform measurements are applied to the correct flip-flop.
Applicant has discovered that clamp type pickups do not always adequately distinguish one cylinder from another. For example, a passive inductive pickup was found to be sensitive to more than one cylinder firing, therefore not useful for cylinder identification. The present invention substitutes for such a passive inductive pickup a direct hardwire connection to the high tension electrode in the No. 1 cylinder. The pickup is preferably designed to produce an output pulse of suitable magnitude and width to trigger a flip-flop or other readout device.